i) Field of the Invention
The present invention relates to a method of producing nanocrystalline cellulose (NCC) films by moderate heating of NCC suspensions and to producing patterns incorporated into the structure of the NCC films by controlling transfer of heat to the suspensions by means of materials having thermal properties different from those of the heating environment. The invention also relates to iridescent solid nanocrystalline cellulose films incorporating patterns.
ii) Description of the Prior Art
Cellulose is the most abundant organic compound on earth. It is the structural component of the primary cell wall of higher plants and green algae, and it is also formed by bacteria, some fungi, and tunicates (invertebrate marine animals) [1].
Native cellulose has a hierarchical structure, from the polymeric glucose chains to the microfibrils which make up the cell walls of plants. The cellulose polymer chain is derived from D-glucose units, which condense through β(1→4)-glycosidic bonds giving a rigid straight chain having many inter- and intramolecular hydrogen bonds among the many glucosidic hydroxyl groups. These features allow the cellulose chains to pack closely to give areas of high crystallinity within the microfibril [2]. Cellulose microfibrils also contain amorphous regions randomly distributed along their length [3-5].
Cellulose whiskers or nanocrystals are obtainable by controlled acid hydrolysis of the cellulose sources listed above, in particular from wood pulp and cotton. The less-dense amorphous regions along the cellulose microfibril are more susceptible to acid attack during hydrolysis and cleave to give cellulose nanocrystals [6,7]. Their low cost, renewability and recyclability, and their chemical reactivity allowing their chemical and physical properties to be tailored make nanocrystalline cellulose whiskers attractive for various applications [8,9].
Nanocrystalline cellulose (NCC) is rodlike in shape with an aspect ratio which varies from 1 to 100 depending on the cellulose source. Wood cellulose nanocrystals average 180-200 nm in length with a cross section of 3-5 nm [9]. Nanocrystal dimensions also depend to a certain extent on the hydrolysis conditions used to obtain them.
The stability of NCC suspensions derives from sulfate ester groups imparted to the cellulose nanocrystal surfaces during hydrolysis with sulfuric acid. The NCC particles are therefore negatively charged in aqueous media and are thus electrostatically stabilized [7, 10-14]. Hydrochloric acid has also been used to produce NCC, but does not introduce charged surface groups [15].
The anisometric rod-like shape and negative surface charge of NCC particles result in suspensions which phase separate into an upper random phase and a lower ordered phase, at concentrations above a critical concentration, as described theoretically by Onsager [16]. The ordered phase is in fact a liquid crystal; liquid crystalline behaviour of cellulose suspensions was first reported by Rånby in 1951 [10]. Marchessault et al. and Hermans demonstrated that such suspensions displayed nematic liquid crystalline order [11,17]. In 1992, Revol and co-workers showed that the suspensions in fact formed a cholesteric, or chiral nematic, liquid crystalline phase [12].
As shown in FIG. 1A, between two critical concentrations, an NCC suspension will separate into two phases [16]. This region spans a range of approximately 1-15% (w/w) for cellulose nanocrystals, depending on the cellulose source. As the NCC concentration increases, the volume fraction of liquid crystalline phase increases until the suspension becomes completely chiral nematic above the upper critical concentration. As shown in FIG. 1B, chiral nematic liquid crystals contain rods arranged in pseudo-layers [18,19]. The rods are aligned parallel to each other and to the plane of the layer, each layer being rotated slightly with respect to the layers above and below it, thereby producing a helix composed of the pseudo-layers. The pitch P of the helix is defined as the distance required for the NCC particles to make one full rotation about a line perpendicular to the layers.
As disclosed in U.S. Pat. No. 5,629,055, aqueous NCC suspensions can be slowly evaporated to produce solid semi-translucent NCC films that retain the chiral nematic liquid crystalline order which forms above the critical concentration and increases in volume fraction as the water continues to evaporate [20,21]. These films exhibit iridescence by reflecting left-handed circularly polarized light in a narrow wavelength band determined by the chiral nematic pitch and the refractive index of the film (1.55) according to Equation 1:λ=nP sin θ  (1)where λ is the reflected wavelength, n is the refractive index, P is the chiral nematic pitch, and θ is the angle of reflection relative to the surface of the film [21]. The wavelength reflected thus becomes shorter at oblique viewing angles. This reflectance was explained by de Vries [22] on the basis of Bragg reflections in a helicoidal arrangement of birefringent layers, as is the case for cellulose nanocrystals in a chiral nematic liquid crystal. When the pitch of the helix is on the order of the wavelengths of visible light (around 400 to 700 nm), the iridescence will be coloured and will change with the angle of reflection. It has been found that the iridescence wavelength can be shifted toward the ultraviolet region of the electromagnetic spectrum by increasing the electrolyte concentration (e.g., NaCl or KCl) in the NCC suspension prior to film formation [21]. The additional electrolyte partially screens the negative charges of the sulfate ester groups on the NCC surfaces, reducing the electrostatic repulsion. The rodlike particles therefore approach each other more closely, which reduces the chiral nematic pitch of the liquid crystal phase and therefore shifts the film iridescence to shorter wavelengths. This method of “blue-shifting” NCC film iridescence is limited by the amount of salt which can be added before the suspension is destabilized by too much screening and gelation occurs [13,21].
The NCC film iridescence colours observed by Revol et al. (1998) also depended on the cellulose source and the hydrolysis conditions (e.g., reaction time and ground cellulose particle size). Smaller NCC particles yield films with a smaller pitch. Desulfation by heating the suspensions prior to forming the films was also found to reduce the chiral nematic pitch [21].
The microstructure of solid NCC films depends on the drying conditions [23]. Suspensions evaporated at ambient conditions generally produce films with polydomain structures in which the helical axes of different chiral nematic domains point in different directions. Drying NCC suspensions in a strong (2 to 7 T) magnetic field will align the axes to produce a more uniform texture, increasing the intensity of the iridescence without changing the wavelength [21,24].
In the laboratory-scale procedure for producing NCC, sonication is used as a final step following acid removal by dialysis, in order to disperse the particles to obtain a colloidal suspension [13,24]. The effects of sonication on NCC suspension properties have been studied by Dong et al. [14]. They found that brief sonication was sufficient to disperse the cellulose particles and further sonication was counterproductive. A more recent study corroborates this observation [25]. Sonication is thought to break up side-by-side NCC aggregates in suspension [7].
Films of NCC have also been prepared on substrates such as silicon [26]. These films are much thinner than the solid NCC films and are composed of alternating layers of NCC and a cationic polymer (poly(allylamine hydrochloride)). Above a certain thickness, the films exhibit colours that change with increasing thickness, but these colours are due to destructive interference between light reflected from the air-film interface and from the film-substrate interface [26]. Interference colours have also been seen in polyelectrolyte multilayers of microfibrillated cellulose [27].
The sulfate ester groups are associated with H+ counterions from the acid hydrolysis, which can be neutralized with a range of bases (MOH) to give salt forms of NCC, (M-NCC) with neutral counterions other than H+, such as alkali metals and in particular Na+, K+ or Li+, or organic phosphonium (R4P+) and organic ammonium ions (R4N+), where each R group which may be the same or different from the other R groups, is an organic chain or group, for example a phenyl group or an alkyl chain of 1 or more, preferably 1 to 4, carbon atoms (e.g., tetraethylammonium ion, (C2H5)4N+) [28]. The acidic NCC is designated H—NCC, while the neutral sodium form of NCC is designated Na—NCC. Thermal treatment, both gentle and harsh, has been used to stabilize NCC films, dried by evaporation, against redispersal in water: heating at 35° C. for 24 h in a vacuum oven is sufficient for solid H—NCC films evaporated at ambient conditions [28], although heating overnight at 105° C. [29] and at 80° C. for 15 min [30] have also been used to stabilize spin-coated H—NCC films. NCC films containing Na+ counterions have been stabilized at 80° C. for 16 h [20]; as well as at 105° C. for times between 2 and 12 h.
Prior to the present invention, there has been no method to produce films of NCC containing patterns incorporated into the film structure.